(a) Field of the Invention
The present invention relates to a power control techniques, and in particular, to power factor correction circuit and an output voltage control method thereof.
(b) Description of the Related Art
Most switching mode power supplies (SMPSs), such as the EN61000-3-2, use a power factor correction circuit because of the current harmonic rule. An SMPS converts an input voltage into at least one DC output voltage and is typically used, for example, in a mobile telephones and laptop computers. A power factor correction circuit is used in the SMPS to correct the power factor by controlling an input current to follow an input voltage. That is, the power factor correction circuit controls the input current to follow the external input voltage, and concurrently converts an input AC voltage into a constant DC voltage.
Most power factor correction circuits use a boost circuit. In general, the power factor correction circuit is manufactured to cover a wide range of voltages between 85Vac and 265Vac since the AC voltage supplied to home appliances is normally between 110Vac and 220Vac. The boost circuit used for the power factor correction circuit is designed such that the output voltage may be greater than the input voltage, and hence the output voltage of the boost circuit is to be substantially close to 400Vdc such that the boost circuit may be operable when the input voltage ranges from 85Vac to 256Vac.
However, an undesired switching loss occurs when the output voltage is designed to be 400V while the input voltage is low. The voltage at a switch when the switch is turned off in the power factor correction circuit is defined to be the output voltage, and an undesirable switching loss by the switch occurs when the output voltage is high (i.e., 400Vdc), even though a low input voltage is given. To solve this problem, the power factor correction circuit adopts the method for varying the output voltage according to the input voltage.
U.S. Pat. Nos. 5,349,284 and 6,686,725 disclose methods for varying the output voltage according to the input voltage. U.S. Pat. No. 5,349,284 discloses a method for detecting the peak value of an input voltage, varying a reference voltage compared to a corresponding output voltage, and thus varies the output voltage according to the input voltage. U.S. Pat. No. 6,686,725 discloses a method for using two output voltages according to the range of an input voltage. That is, the output voltage is set to be 220Vdc when the input voltage varies from 85Vac to 150Vac, and the output voltage is set to be 400Vdc when the input voltage varies from 150Vac to 265Vac.
FIG. 1 is a schematic diagram of a conventional power factor correction circuit disclosed in U.S. Pat. No. 6,686,725. In this circuit, when the input voltage ranges between 85Vac and 150Vac, the Zener diode ZD does not exceed the breakdown voltage and is turned off. The transistor Q1 is turned off as well. Therefore, the voltage input to an inverting terminal of (−) of a comparator 5A is generated by dividing the voltage of Vout by resistors 6 and 7, and the output voltage is established to be 220Vdc. However, when the input voltage ranges between 150Vac and 265Vac, the Zener diode ZD exceeds the breakdown voltage and is turned on, and the transistor Q1 is accordingly turned on. Therefore, the voltage input to the inverting terminal of (−) of the comparator 5A is decreased by dividing the voltage of Vout by the resistors 6 and 7 and a resistor R7, and the output voltage is established to be 400Vdc.
However, as shown in FIG. 1, the power factor correction circuit of U.S. Pat. No. 6,686,725 requires a plurality of external elements—e.g. diodes D1, D3, capacitors C1, C2, ZD, resistors R6, R7, switch for sensing the input voltage such that the output voltage can be varied according to the input voltage. These additional elements increase the overall costs. Similarly, a circuit disclosed in U.S. Pat. No. 5,349,284 also requires many external elements for sensing the input voltage, which also increases overall costs.